The present invention relates generally to a method for changing widthwise distribution of thickness of a metal strip and, more particularly, to a method for correcting cross-sectional profile such as crown, edge drop or the like and flatness of a strip of such metal as steel, copper, aluminum, or the like, and for working the metal strip into a special sectional profile.
Generally, flatness of a rolled material such as a metal strip or the like denotes a shape such as center buckle, wavy edge, waving or the like and overall lengthwise or widthwise curve, crown of a rolled material denotes a profile of a cross section of the material, and the thinning inclination in the widthwise edges of the material is particularly referred to as edge drop.
In recent years, the requirements for accuracy of flatness and crown of rolled products such as steel strip and the like have become very strict. In conventional rolling mills, while the lengthwise thickness of rolled material can be controlled relatively accurately by an automatic thickness control device, control of the widthwise thickness of the material is limited since it is performed by a roll bender.
Accordingly, as means for increasing accuracy of crown and shape of the material in place of the roll bender, a rolling mill having axially movable rolls and a rolling mill having variable-crown rolls have been developed. The former rolling mill has six-high rolls, intermediate rolls of which are axially oppositely movable to control the backing up force of the work rolls. The latter rolling mill comprises a pressure receiving chamber between the arbor and the sleeve of a roll, into which a pressure medium is introduced to regulate the pressure of the medium, thereby controlling the value of crowning of the roll.
These conventional techniques are means for improving rolling mills to increase the capacity of controlling the roll deflection and for minimizing the crown of the material (rather small in quantity, that is in the range of 0.5% to 2% of the thickness). However, these conventional techniques have limit and are unable to produce perfectly rectangular section. Further, decreasing of crown of the material causes wandering of the material which presents a problem of unstable operation. Furthermore, in the rolling mills, while crown of the material can be reduced to some extent since the deflection of the rolls is reduced, it is impossible to eliminate the edge drop. Furthermore, reconstruction of the rolling mill requires a large amount of investment.
An inexpensive method for correcting crown of material which can be conceived is to reduce the crowned portion of the rolled material by light reduction rolling to control the section of the rolled material into a rectangular shape. In this method, however, the shape of the rolled material will be extremely worsened to make the rolling operation impossible.
Edge drop will now be described. Generally, a rolled material shows a tendency of a sudden decrease in thickness in the region of 10 mm to 50 mm from the edges thereof. This edge drop is considerably large and, for example, in a hot strip of steel, it can be as large as 0.1 mm. Therefore, in steel sheets such as hot strip and cold strip, it is frequently necessary to trim edges or neighborhood thereof, thereby causing decrease in yield.
It can be said that the existing production lines are left as they are, without any particular countermeasure being considered against the edge drop.
While several studies have been done to reduce the edge drop by devising a rolling method, they are, as will be explained in detail, technically ineffective or difficult to reduce to practice.
As another method for reducing edge drop, use of work rolls of a smaller diameter has been proposed. The purpose of this method is to reduce the flattening of the roll by reducing the rolling load, to thereby reduce the edge drop. In this method, however, the line speed is decreased and the unit consumption of the roll is worsened. Further, the effect of this method is small because it is impossible to eliminate edge drop completely.
As still another method, use of work rolls with tapered ends has been proposed. The purpose of this method is to decrease the reduction of the edge portions of the rolled material by providing the work rolls with taper. In this method, however, it becomes necessary to change rolls every time the width of the material is changed and the desired effect is not obtained when wear of the rolls proceeds.
A tension leveler closely connected to the present invention will now be described. Shape defect, such as center buckle and wavy edge, of material are due to difference in lengthwise elongation of the material and, accordingly, cannot be corrected unless the material is provided with plastic elongation.
The tension leveler is designed to correct shape defects by providing plastic elongation to the material with bending under tension. The difference in elongation of the material by a rolling mill is usually at most in the order of 0.1%, by which shape defects such as center buckle and wavy edge are produced. By elongating this to the order of 0.2% to 0.5% by the tension leveler the material is flattened. The conventional tension levelers are designed to provide the maximum percentage of elongation in the order of 1% to 1.5%. Since it is used usually in the lowest required percentage of elongation, reduction in thickness or width of the material by the tension leveler is very small and practically neglected. In the conventional method, after all, there is no idea of correcting crown of the rolled material using the tension leveler.